Serum binding of steroid tracers and its possible effects on direct steroid immunoassay.

We studied the serum protein binding of 3H-labelled progesterone, oestradiol and testosterone, and five 125I-labelled analogues of these steroids. All tracers investigated appeared to be bound by proteins in every serum sample tested. The addition of blocking agents caused a substantial reduction in serum protein binding of 3H-labelled steroids, but had relatively little effect on the binding of analogue steroid tracers. Use of analogue steroid tracers in conventional direct immunoassays for oestradiol and progesterone produced anomalous results for some patient samples when compared to extraction radioimmunoassays, but assays where tracer binding to serum constituents was prevented by adoption of two-step procedures appeared to avoid anomalous results. The results suggest that serum protein binding of steroid analogue tracers may be a source of interference in some direct steroid immunoassays.